Would VFD on Secondary Chilled Water Pump Cause Low Delta_T Symptom 1

[yyj10]

Hi Guys, I am doing a energy conservation project and trying to add a VFD on the secondary and tertiary pumps in a primary/secondary chilled water system. But I am a little concerned about chiller low delta_T symptom when the cooling load is low in some cool days. Anyone has any experience on this? Thanks so much!

 

[KiwiMace]

No, a vfd would improve it, and there should be a vfd on a primary/secondary distribution anyway.

There is a ream of information available on this from a variety of sources. Start with the ASHRAE systems and equipment handbook and keep on reading.

 

[yyj10]

Thanks a lot, KiwiMace!It is standard to put VFD on secondary pumps. I just find out that this does not solve low delta_T symptom, if the system is experiencing it. But, for this case, the chiller size is properly chosen. I believe that we will not suffer low delta_T symptom as far as we put right control on VFD.

 

[tys90]

It's very hard to eliminate low delta_T syndrome. The best action is to take the correct steps to minimize the efficiency losses that come with low delta_T. As KiwiMace said, there are numerous articles on the internet and in print that address low delta_T with all types of pumping and chiller systems.

 

[walkes]

I don't believe that adding VFDs to secondary and tertiary pumping alone will address the low delta T issues. The plant needs to be able to be staged to match the load as best it can, including flow rate.

If your primary loop is constant flow with a constant discharge temperature, the Delta T is going to vary with the secondary load not just the flow.

If you can vary the primary loop flow to match your load, you should be able to reduce low delta T issues.

 

[yyj10]

Thanks walkes! Right, delta T is going to vary with secondary load. A VFD chiller is the best option to solve this problem. So far, the delta T flutuation is acceptible in the design range.

 

[pretendfarmer]

You cannot solve Low Delta T in the chiller plant. You can add VFD's on your secondary (distribution) pumps but if not properly controlled you will still have Low Delta T. I have seen a lot of systems that add VFD's to all the chilled water pumps and still have Low Delta T issues.
All Low Delta T means is that you are satisfying the load with a flow rate that is too high. A properly controlled hydronic chilled water system seldom if ever requires more than 50-60% of design coil flow to satisfy the system. I know of one project in particular that the owner paid millions of dollars to replace his chilled water plant and still ended up with Low Delta T (running too many chillers to satisfy the load). This is because he tried to solve the problem in the chiller plant and not at the air handlers. Fortunately for the owner, the project was a performance contract. The performance contractor was on the hook and ended up spending an extra million out of pocket to fix the problem.

It all boils down to proper control at the air handlers. Properly controlled, the air handler should never operate below it's design Delta T. Most systems operate below design Delta T because the control valve is not capable of stable control at less than design flow. It is not impossible. In fact, there are designs that guarantee solving Low Delta T in chilled water systems. I work with them everyday.

 

[Drazen]

VFD pumps on secondary with hydronic decoupler installed as something understandable by itself should not have any adverse effect on primary, the only visible consequence is that return temperature will have less oscillation.

The question is whether you believe that such action itself will help low delta t, but as far as I can understand, you decided on VFD pumps for different reason and just want to check about low delta t.

 

[Waramanga]

what does the system currently have to regulate the secondary flow? Is it just a bypass in the plantroom running on a dP sensor? I cant see how adding a VSD on the secondary pumps could reduce the dT at the chillers, I can only see it improving it (water will return from the field warmer at part loads given that a coils dT goes up at part load).

I guess the question is how do you make sure that the chillers are staging correctly. I have seen flow meters in the bypass lines of primary secondary pumping. I think that when the bypass is running backwards you know that the field is asking for more chilled water than you are producing and its time to bring on another chiller. Does anyone know about this strategy and have a good reference describing how it works??

 

[BronYrAur]

I keep these 2 articles handy when the subject comes up. There are of course a host of others

 

[BronYrAur]

Looks like only 1 article got attached on my last post. Here is the other.

 

[DRWeig]

Good on ya, BronYrAur. You beat me to it.

Good on ya,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[317069]

you want to use VFD on secondary but you didn't say what about primary,?
if the secondary flow rate was less than primary, then a part of supply water will go back through common pipe and mix with return water at primary inlet and that would decrease the return water temperature

 

[urgross]

What are you using to control the VFD? How is the coil load being controlled? Are you using temperature reset with the primary? Pretty much conjectural without more information.